Results

Climate in the study period
Monthly variations of temperature and precipitation in the Selva Piana beech forest are reported in Figure 1a-b. In 2016 a severe late frost event occurred during the night between April 25 and 26, when the temperature at canopy level (~ 24 m) reached – 6 °C (Fig. 1a inset panel). The extreme frost event followed an early spring season characterized by a temperature that during the months of February and April was significantly higher (about 2°C) than the average value of the site for the period 1989-2015 (Fig. 1a). In 2017, from May to August, the temperature was significantly higher than the average value of the site, with a positive anomaly of ~3 °C (Fig 1a). Furthermore, from May to October 2017 a significant reduction of precipitation against long term average was observed (Fig. 1b), leading to an annual precipitation that was ~ 50% lower than the 1989-2015 average (Fig. 1b inset panel).
Phenological parameters and radial growth
The seasonal LAI trend, used to define the phenological phases of the stand, is reported in Fig. 2a. The “first” green up in spring 2016 occurred between 20 and 30 days earlier than the average of the site (Fig. 2a), while the “second” (re)green up, after the complete canopy destruction due to the spring frost event, started around June 28, with a leafless period of more than 60 days. In 2016 the beginning of the senescence phase was anticipated of about one week compared to the average of the long-term series (Fig. 2b). Maximum LAI was lower in 2016 (LAI = 4.79 m2 m–2) than in 2017 (LAI = 5.37 m2 m–2), while the long-term average LAI of the site assessed with remote sensing was ~5 m2 m–2 (Fig. 2a). The average length of vegetative period assessed through remote sensing during the 2000-2015 period was approximately 140 days, a value confirmed in 2017, while it was 83 days in 2016.
The mean BAI in the 2000-2015 period was 22.64 ± 0.78 cm2 year–1, while it was 3.69 ± 1.14 cm2 year–1 and 18.75 ± 3.80 cm2 year–1 in 2016 and 2017, respectively (Fig. 2b inset panel). The late frost in spring 2016 reduced the stem radial growth of about 85% compared to the average of the period 1989-2015. The late frost strongly affected the seasonal dynamic of stem diameter growth during the year 2016, as shown by the lower and almost constant rate of stem growth compared to 2017, when after the green up the radial growth followed the usual pattern, reaching the highest increment (32.30 ± 4.14 μm day–1) in July (Fig. 2b).